Federal prosecutors have begun a criminal investigation into a norovirus outbreak at the Chipotle Mexican Grill in Simi Valley that sickened nearly 100 customers and employees in August.

In a filing Wednesday with the Securities and Exchange Commission, Chipotle said it was served with a federal grand jury subpoena in December "in connection with an official criminal investigation being conducted by the U.S. attorney's office for the Central District of California in conjunction with the U.S. Food and Drug Administration's Office of Criminal Investigations."

The subpoena requires the Denver-based company to produce a broad range of documents related to the outbreak, the filing said.

"We intend to fully cooperate in the investigation," Chipotle said in the filing.

The company said it doesn't yet know what, if any, fines and/or penalties it will face as a result of the investigation.

Thom Mrozek, spokesman for the U.S. attorney's office in Los Angeles, also declined to comment.

Doug Beach, a manager of the food program for Ventura County's Environmental Health Division, said Wednesday that federal prosecutors requested records about the outbreak from his office about a month ago.

"That was a first for us," Beach said, adding that the office complied.

Environmental health investigated numerous complaints of food poisoning from people who ate at the restaurant in the Simi Valley Town Center from Aug. 18-19, determining that norovirus was to blame.

"It was probably due to a sick employee or employees," Beach said.

Norovirus is a highly contagious virus that can be gotten from an infected person, contaminated food or water, or by touching contaminated surfaces, Dr. Robert Levin, the county's public health officer, said in September in announcing the results of the county investigation. It can lead to stomach pain, nausea, diarrhea and vomiting.

Beach said Chipotle cooperated with the investigation, which uncovered issues such as unclean equipment and employees without the necessary food handling permits.

But he said that the company arguably did not follow proper protocol by not reporting the complaints about the illnesses to environmental health until Aug. 21. That was after the company reopened the restaurant, which it had closed Aug. 20, he said.

While the state retail food code doesn't specify a time by which a restaurant must notify health officials, "normally, we are informed much earlier than that," Beach said. "We believe they should have notified us sooner, not late in the game.

"Because they had already shut the restaurant down, thrown out all the food, completely cleaned the place up, went top to bottom with bleach, and brought in a new staff, we didn't have any opportunity to sample food or do some of the things we normally do to investigate a foodborne outbreak," he said.

Beach said the county doesn't "know exactly what the restaurant did between the 18th and the 21st as far as who they knew was sick on their staff, did they send them home, did they keep them there? Had we known earlier, we potentially could have prevented more people from getting sick."

Arnold declined to comment on Beach's remarks.

Seattle-based food safety attorney Bill Marler, who represents about two dozen of the customers who got ill, said he's never heard of a federal criminal investigation into any kind of restaurant outbreak.

"So I am at somewhat of a loss as to why they would investigate this particular restaurant," said Marler, who has filed two federal lawsuits against Chipotle over the illnesses.